From the muscles allowing people to run to those that direct the blink of an eye, skeletal muscles come in a variety of shapes and sizes. These muscles have different functions;additionally, they also have different susceptibilities to diseases like muscular dystrophy and rhabdomyosarcoma. Since potential therapies to repair muscle wasting will require the ability to generate muscles of a specific size and shape, an important step in developing treatments will be to understand how muscle diversity is achieved. In the fruit fly Drosophila melanogaster, a group of transcription factors, known as identity genes, mark subsets of muscles and are important for their morphology. Few downstream targets of these transcription factors are known, however, and little is known about how identity genes control muscle diversity. The goal of this proposal is to determine how morphological information is translated from the identity genes to the cellular processes that control muscle size and shape. The central hypothesis of this research is that identity genes work in combination with each other to direct muscle morphogenesis by regulating cytoskeletal remodeling, cell adhesion and myoblast fusion, among other cellular behaviors. The morphogenesis of the lateral transverse (LT) muscles of the Drosophila embryo will be analyzed using mutant analysis, live imaging, and genomic approaches to identify factors regulating the development of those muscles. The focus of these experiments will be to determine how identity genes work in combination to direct LT muscle morphology. Five identity genes have been shown to be expressed in and affect morphogenesis of the LT muscles: Kruppel, twist, apterous, muscle segment homeobox and caupolican. Mutants for each of these genes will be examined to determine the relative contributions of each identity gene to the final muscle morphology, as well as to determine the genetic hierarchy beteween these five transcription factors. Morphogenesis will be characterized by live confocal imaging of fluorescent markers, elucidating the steps in this process and aiding in the interpretation of genetic data. Finally, fluorescence activated cell sorting (FACS) will be used to isolate subpopulations of myoblasts from Drosophila embryos;microarray analysis will be performed on particular groups of cells to identify the targets of identity genes within the developing muscles. We will focus further studies on those genes predicted to have roles in cell shape or tendon attachment. Taken together, these data will provide insight to the mechanisms by which identity genes act together to specify a muscle's unique size and shape.